The picosecond thermoreflectance method is a powerful method to measure a thermal diffusivity of a thin film which is thinner than one micrometer. FIG. 2 illustrates the principle of the picosecond thermoreflectance method. To irradiate an extremely fast light pulse, as a heating light, to a boundary between a substrate and a thin film will cause an instantaneous temperature rise of the boundary, and thereafter the heat diffuses inside the thin film. In order to observe the temperature response of the surface of the thin film, the picosecond (or femtosecond) thermoreflectance method irradiates a probe light pulse onto the surface of the thin film, and observes the surface temperature change of the thin film from the reflectivity change of the probe light that depends on the temperature change. The temperature change per one pulse is about 0.1° C., and the reflectivity change proportional to this temperature change is as small as about one hundred-thousandth.
FIG. 3 illustrates a block diagram of the conventional thermoreflectance method using a picosecond (or femtosecond) pulse laser. The pulse laser 101 emits a light pulse L101 of which pulse width is 2 picoseconds with a repetition frequency of 76 MHz, and a beam splitter 103 splits the light pulse L101 into a heating light L103 for a sample and a probe light L105.
The intensity of the heating light is modulated by an acousto-optic modulator 107 with 1 MHz, the heating light passes a delay line R101 and heats the surface of the sample. A modulation signal generator 111 generates a signal inputted to the acousto-optic modulator 107, and delivers a part of the output to the reference signal input of a lock-in amplifier 115.
The delay line R101 is made up with a mechanism 121a in which a corner cube retroreflector 121b moves in parallel. By moving the position of the corner cube retroreflector 121b, the method adjusts the time for the probe light pulse L105 reaching the sample against the heating light pulse L103. The modulated heating beam travels through an optical delay line. The corner cube retroreflector can translate over the distance of about 150 mm. To move the corner cube retroreflector 121b by 0.15 mm will change the optical path length by 0.3 mm, which corresponds to one picosecond in terms of the arrival time difference between the pump light pulse and the probe light pulse.
The probe light L105 is focused on the back of an area where the heating light L103 is focused, as shown in FIG. 2, and a reflected light thereof falls on a detector 123 (FIG. 3). A detected signal is led to the signal input of the lock-in amplifier 115.
Since the reflectivity change proportional to the temperature change is as small as one hundred-thousandth, the lock-in-amplifier 115 detects the components synchronized with the modulation frequency 1 MHz from the detected signal. FIG. 4 illustrates the principle of detecting the temperature response by pulse heating. When the heating light pulse is repeatedly emitted, the surface temperature of the sample changes with the same repetition frequency. If the probe light pulse repeatedly emitted with the same frequency reaches the sample surface with a delay of the time difference t against the heating light L103, it will give an intensity change of the probe light L105 that is proportional to the temperature change after t second from the pulse heating. Besides, since the intensity of the heating light is modulated by 1 MHz, the intensity of the reflected probe light also varies with 1 MHz. To detect the temperature change at a certain delay time t, the components synchronized with the modulation frequency 1 MHz by the lock-in-amplifier 115 is recorded using a computer system 117. The thermoreflectance history curve by pulse heating can be acquired by varying the delay time t of the probe light L105 to the heating light L103.
The picosecond thermoreflectance method is a powerful method in order to observe the thermal properties of a thin film, however there still remains the following problems.
1. When the optical path length is changed, the irradiated area changes due to beam divergence. To move the optical path length by more than 30 cm (equivalent to the time difference of 1 nanosecond) will change the spot area by 10 micrometer, which leads to a drift of the thermoreflectance signal detected by the lock-in-amplifier. From this limitation, the difference in the optical path length has to be within about 30 cm.
2. In case of a thin film material with more than 100 nanometers in thickness, a thin film of low thermal conductivity, and a multi-layered film material having a high boundary thermal resistance, there are some materials that require more than one nanosecond for the temperature to be transmitted from the back to the surface. In such a case, even if the optical path length is moved to the maximum, it is impossible to confirm the steady state of temperature rise by one pulse, which results in difficulty of the quantitative measurement of the thermophysical properties.
In order to carry out the thermal design of semiconductor devices, large-capacity storage media such as optical disks, magneto-optical disks, hard disks, and so forth, or to understand the transport phenomena of the thermal energy in the highly technological multi-layered film such as laminated composite materials, it is essential to know the thermophysical properties of the each layer and the boundary thermal resistances. The conventional picosecond (or femtosecond) thermoreflectance method is likely to be influenced by a slight dislocation of the optical axis that is created in measurement. In case of measuring the thermophysical properties of a comparably thick material (more than 100 nanometers), a multi-layered film material, a low thermal conductivity material, and so forth, which require a comparably long time for the transmission of the thermal energy, it takes more than one nanosecond for the temperature rise on the back side. Accordingly, the method does not present the total understanding of the temperature response, and gives difficulties in the quantitative measurements of thermal diffusivities and boundary thermal resistances.